It has been shown that inattentional blindness, omission of the unexpected extra item once attention is engaged in some other task, is dependent on perceptual load (Cartwright-Finch & Lavie, 2007) and attention set (Most, 2011). Here, we were interested in the question on how the extra item is processed during a multiple identity tracking task, a real-life task, where sustained attention is engaged to distinct moving identities (e.g Oksama & Hyönä, 2008). We manipulated the familiarity of the attention set (whether the tracked identities were repeated or varied between the trials), task load (the number of targets and distractors from 2 to 4) and the presence of an extra item during the tracking task. Besides doing the tracking task, participants also counted the bounces of the targets by pressing the keyboard continuously during a trial. We measured the accuracy of target, distractor and extra item recognition, the accuracy of the bounce calculation and participants' eye-movements during the task. We found that the rate of the extra item omission increased when tracked identities were less familiar and when the target or distractor set-size increased. Furthermore, the bouncing accuracy decreased in the extra-item-present trials compared to extra-item-absent trials. Fixation duration, number of fixations and saccade velocity were sensitive to the extra item whereas pupil size' sensitivity was mediated by number of factors. In sum, the extra item appearance increases perceptual/cognitive load of the task. The processing of extra items during multiple identity tracking is discussed in the light of the present results.